1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus such as an electrophotographic copier, a printer, a facsimile, and a multifunction peripheral having functions of those devices, and to an image forming apparatus provided with the developing device.
2. Description of Related Art
In image forming apparatus, an electrostatic latent image formed on a photosensitive member is developed by a developing device and visualized as a toner image. As an example of the developing device, there has been practically used a small-sized and inexpensive developing device using a magnetic one-component developer. The developing device is provided with a fixed magnet body having a plurality of magnetic poles provided in a developing sleeve serving as a toner carrier and a regulating member for regulating an amount of toner on the developing sleeve. With this, a predetermined amount of a toner thin layer is carried on the developing sleeve between the regulating member and the developing sleeve.
As an example of the developing device just described above, there has been well-known a developing device according to a first related art in which the toner thin layer is uniformly formed on the developing sleeve.
In the developing device according to the first related art, the regulating member is provided at a predetermined interval with respect to the developing sleeve, and includes a magnet constituted by a plate-like blade made of a magnetic material. A magnetic pole of the magnet, which is on a facing side with respect to the developing sleeve, has the same polarity as that of a facing magnetic pole of the fixed magnet body in the developing sleeve, and is attached on an upstream of the blade in a rotational direction of the developing sleeve. A magnetic force of the magnet causes a distal end portion of the blade to be magnetized in reverse polarity to that of the magnet. With this structure, charged toner is conveyed in a state of adhering to the developing sleeve, and thinned by being regulated in layer thickness by the distal end portion of the blade of the regulating member. In this case, a magnetic field is formed by the fixed magnet body and the magnet between the distal end portion of the blade and the developing sleeve, and a magnetic field is formed also between the distal end portion of the blade and the magnet. Those magnetic fields allows the toner to pass between the distal end portion of the blade and the developing sleeve in a substantially uniform state, with the result that a toner thin layer is formed on the developing sleeve.
However, in the developing device according to the first related art, the magnetic forces of the magnet of the regulating member and the fixed magnet body in the developing sleeve are liable to be smaller on end portions than those on central portions in longitudinal directions thereof. Thus, between the regulating member and the developing sleeve, there is a risk that magnetic fields are weakened on the end portion sides in comparison with the central portions in the longitudinal directions. When the magnetic fields are weakened on the end portion sides, a toner regulating force on the end portion sides is lowered. A toner charging amount on the developing sleeve increases in accordance with rotation of the developing sleeve. When the toner charging amount increases, toner firmly adheres to the developing sleeve, and toner particles adsorb to each other. As a result, after repeated development, at the time of passing between the regulating member and the developing sleeve, even when the toner has a predetermined layer thickness on the central portion in the longitudinal direction, a toner layer thickness increases on the end portion sides owing to weakness of the toner regulating force. As a result, disturbance of the toner layer is liable to occur. When the toner is supplied to a photosensitive member under a state in which the disturbance of the toner layer markedly appears and the toner layer thickness is uneven, there is a problem in that satisfactory toner images are not formed on the photosensitive member.
Under the circumstance, in order to overcome the disturbance of the toner layer just described above, in a developing device according to a second related art, there is provided a magnetic-field generating member constituted by a magnet. On an upstream in the rotational direction of the developing sleeve with respect to the regulating member, the magnetic-field generating member is provided at a position facing inter-magnetic-pole portions of the fixed magnet body in the developing sleeve so as to form a magnetic brush of the toner at the position. With this structure, residual toner still remaining on the developing sleeve after development is scraped off from the developing sleeve by the magnetic brush. Then, toner is re-carried on the developing sleeve, and the toner carried thereon is conveyed to a regulating member side.
In the developing device according to the second related art, an excessively large magnetic field of the regulating member causes a thickness of the toner layer formed on the developing sleeve to be small, and hence an amount of toner supplied from the developing sleeve to the photosensitive member is reduced. As a result, sufficient image density cannot be obtained. As a countermeasure, it is necessary to set the magnetic field of the regulating member to have a predetermined value so that toner has an appropriate layer thickness. When toner caused to adhere by the magnetic field of the regulating member is scraped off with use of the magnetic-field generating member after development, an excessively large magnetic field of the magnetic-field generating member causes the toner to be subjected to stress, with the result that toner characteristics such as charging characteristics are deteriorated. Meanwhile, an excessively small magnetic field of the magnetic-field generating member causes the adhering toner to be left on the developing sleeve. Thus, after repeated development, the toner layer thickness varies in the longitudinal direction, and the disturbance of the toner layer markedly appears. In addition, when the toner adheres on the developing sleeve and left thereon, there is a problem in that a development ghost is generated owing to a difference in charging amount between the adhering toner and newly supplied toner.
Further, in the developing device according to the second related art, when the magnetic-field generating member constituted by a magnet is extended in the longitudinal direction of the developing sleeve and attached to the developing container, there is a risk that the interval between the developing sleeve and the magnetic-field generating member cannot be uniform in the longitudinal direction depending on a dimensional accuracy between the magnetic-field generating member and a member such as a developing container or a dimensional accuracy between the developing sleeve and the magnetic-field generating member. Such non-uniformity of the interval causes the magnetic field between the developing sleeve and the magnetic-field generating member to be unstable, and hence toner on the developing sleeve cannot be sufficiently scraped off.
Further, in the developing device according to the second related art, in order to sufficiently scrape off the toner layer having a large thickness on the end portion sides on the developing sleeve, it is necessary to increase the magnetic force of the magnetic-field generating member or to reduce the interval between the magnetic-field generating member and the surface of the developing sleeve. In this way, the magnetic force thus increased or the interval thus reduced makes it possible to scrape off the toner on the end portion sides on the developing sleeve. However, the toner layer thickness is relatively small on the central portion side, and the toner layer thickness varies in the longitudinal direction. Thus, when passing the interval between the magnetic-field generating member and the developing sleeve, there is a problem in that the toner is deteriorated by being subjected to stress, and in that toner particles aggregate and damage the surface of the developing sleeve. Further, image failures of vertical streak occur.